Police in Britain said Saturday that they had arrested an 18-year-old man in connection with Friday’s explosion aboard a London subway train that injured 29 people.

Police say the man was arrested by Kent police in the port area of Dover on the English Channel.

Deputy Assistant Police Commissioner Neil Basu said it was a “significant arrest.” He said the investigation was ongoing and the terror threat level remained at “critical.”

The suspect was being held for questioning under the Terrorism Act. He has not been charged or identified.

investigators in Britain have been working feverishly since the attack occurred at the Parsons Green station to collect evidence and information about the attack.

Hundreds of officers were examining surveillance footage and conducting other investigations as the nation elevated its terrorism alert system to the highest level, the BBC reported.

Authorities said the suspect carried a white bucket containing an explosive onto the rush-hour train. When it exploded, numerous train riders suffered burns, and others were injured as they rushed away from the area of the blast.

The Islamic State group claimed responsibility for the attack.

Meanwhile, President Donald Trump took heat from British authorities for a tweet he posted in the wake of the attack that seemed to criticize Britain’s security forces.

“Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!,” the president tweeted.

Reuters reported that British Prime Minister Teresa May responded: “I never think it’s helpful for anybody to speculate on what is an ongoing investigation.”

Prior to the suspect’s arrest, London was on edge. Images from cameras inside the subway car showed that the device was contained in a bucket with wires hanging out of it and that it was concealed in a plastic shopping bag.

Officials have hinted that more than one person may have been involved.

May said raising the threat level to its highest point was a “proportionate and sensible step.” Police called on the public to be vigilant.

The bomb went off around 8:20 a.m. as the train, carrying commuters from the suburbs — including many school children — was at Parsons Green station in the southwest of the city.

The station was reopened Saturday, officials said, restoring some normalcy to London’s transport network after a day of severe disruption. There was no sign of panic among Londoners and the weekend life of the city continued undeterred by the raised threat level.

Officials said the bomb was intended to do grave harm to commuters. Analysts said the injuries would have been far worse had the entire device exploded.

“They were really lucky with this one. It could have really become much worse,” said terrorism specialist Magnus Ranstorp of the Swedish Defense University.

Britain has endured four other attacks this year, which have killed a total of 36 people. The other attacks in London — near Parliament, on London Bridge and near a mosque in Finsbury Park in north London — used vehicles and knives.

In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people. That attack in May also briefly caused the threat level to be set at “critical.”